BIOL 1400 PRACTICE PROBLEMS IN GENETICS - University of Central Arkansas

BIOL 1400

Biology for General Education

PRACTICE PROBLEMS IN GENETICS

Questions 1-12 have to do with domestic cats. However, the same basic principles

will apply (usually), no matter what animals or plants you¡¯re working with.

1. Short hair (L) is dominant to long hair (l). What are the possible genotypes of a

shorthaired cat?

LL or Ll.

2. Suppose you have a shorthaired cat, and you don¡¯t know its genotype. To find

out its genotype, you can breed it with a longhaired cat and study the

phenotypes of the kittens. (Geneticists call this a test cross.) Explain why and

how you could tell your cat¡¯s genotype in this way.

If your shorthaired cat has the LL genotype, then when you breed it

with a longhaired cat (ll), all of the kittens will have short hair (and

all will have the Ll genotype).

If your shorthaired cat has the Ll genotype, then then when you breed

it with a longhaired cat (ll), about half of the kittens will have short

hair (Ll genotype) and half will have long hair (ll genotype).

3. Black fur (B) is dominant to brown fur (b). Suppose you cross two black cats

and some of the kittens are brown. What are the genotypes of both parents?

Both parents must have the Bb genotype. They¡¯re both black cats, so

the must have at least one B allele each (we could write this as ¡°B_¡±.

But if they have brown kittens, the kittens must have the bb

genotype, and the only way that can happen is if each parent

contributes a brown allele. Thus each parent must have the Bb

genotype.

4. Suppose you have two brown cats, which have kittens, and some of the kittens

are black. Why might this make you think that there¡¯s a hole in your fence?

Because this isn¡¯t possible. If both parents are brown, then both

parents must have the bb genotype. But a black kitten must have

received a B allele from at least one parent¡ªand a parent that has a

B allele would have to be a black cat. So some sneaky black tomcat

from the neighborhood must be the kittens¡¯ daddy.

5. Another gene, dilute, has two alleles: D gives a dense, full color to the fur,

whereas d lightens the color¡ªthus a black cat with the dilute trait looks gray,

and a brown cat with the dilute trait looks beige (or ¡°lilac¡±, as breeders say). List

all the possible genotypes that a gray cat could have.

6. You crossbreed two cats, one with the genotype Bbdd and one with the

genotype bbDd. What are the phenotypes of these two parents?

Bbdd = gray cat

bbDd = brown cat

7. What are the expected phenotypes of the offspring, and how many of each

phenotype would you expect to get? Draw up a Punnett square to answer this

question.

Each cat passes on to each offspring one of each pair of alleles of each

gene. The gray cat (Bbdd) can pass on either Bd or bd. The brown cat

(bbDd) can pass on either bD or bd. So. . .

Bd

bd

bD

BbDd

bbDd

bd

Bbdd

bbdd

In other words, about one in four of the kittens would be black

(BbDd), one in four would be brown (bbDd), one in four would be

gray (Bbdd), and one in four would be beige or lilac (bbdd).

8. Some of the kittens that you¡¯d get from the above cross are black. Imagine that

you take one of the black kittens, raise it to maturity, and then mate it with a

lilac / beige cat. What are the expected phenotypes of the offspring, and how

many of each phenotype woul d you expect to get? Draw up a Punnett square to

answer this question.

A black kitten from the above problem has the genotype BbDd. A

beige cat has to have the bbdd genotype. The black cat can pass on

four possible combinations of alleles: BD, Bd, bD, or bd. The beige

cat can only pass on one allele combination: bd. The full-blown

Punnett square looks like this:

BD

Bd

bD

bd

bd

BbDd

Bbdd

bbDd

bbdd

bd

BbDd

Bbdd

bbDd

bbdd

bd

BbDd

Bbdd

bbDd

bbdd

bd

BbDd

Bbdd

bbDd

bbdd

So you get the same answer as problem 7: about equal numbers of

black (BbDd), grey (Bbdd), brown (bbDd), and beige (bbdd) cats.

9. Now imagine that you cross a BbLl cat with a brown

I forgot to complete this problem. I meant ¡°a brown longhaired cat.¡±

Result would be 25% black shorthairs, 25% black longhairs, 25%

brown longhairs, 25% brown shorthairs.

10. Imagine you have a cat with the genotype BbDdLl. What¡¯s its phenotype?

Black and shorthaired.

11. Imagine you crossbreed two cats, each of which has the genotype BbDdLl.

The resulting Punnett square is 8x8, and is a pain in the neck to work out, but if

you can do it you¡¯ve definitely got Mendelian genetics down. What are the

expected ratios of all possible phenotypes in the offspring?

Each parent passes on one of each pair of alleles of each gene. Each

parent can pass on BDL, BDl, BdL, bDL, Bdl, bDl, bdL, or bdl ¡ªeight

possible allele combinations. This gives us an 8x8 Punnett square. . .

BDL

BDl

BdL

bDL

Bdl

bDl

bdL

bdl

BDL

BBDDLL BBDDLl BBDdLL BbDDLL BBDdLl BbDDLl BbDdLL BbDdLl

BDl

BBDDLl BBDDll BBDdLl BbDDLl BBDdll BbDDll BbDdLl BbDdll

BdL

BBDdLL BBDdLl BBddLL BbDdLL BBddLl BbDdLl BbddLL BbddLl

bDL

BbDDLL BbDDLl BbDdLL bbDDLL BbDdLl bbDDLl bbDdLL bbDdLl

Bdl

BBDdLl BBDdll BBddLl BbDdLl BBddll BbDdll BbddLl Bbddll

bDl

BbDDLl BbDDll BbDdLl bbDDLl BbDdll bbDDll bbDdLl bbDdll

bdL

BbDdLL BbDdLl BbddLL bbDdLL BbddLl bbDdLl bbddLL bbddLl

bdl

BbDdLl BbDdll BbddLl bbDdLl Bbddll bbDdll bbddLl bbddll

Rounded off just a bit, this gives you. . .

Black shorthairs: 27/64, or 42%

Black longhairs: 9/64, or 14%

Gray shorthairs: 9/64, or 14%

Brown shorthairs: 9/64, or 14%

Gray longhairs: 3/64, or 4.6%

Brown longhairs: 3/64, or 4.6%

Beige shorthairs: 3/64, or 4.6%

Beige longhairs: 1/64, or 1.5%

Whew. . . No, nothing quite that bad will appear on the final.

12. The dominant allele of yet another gene, called white, blocks all color from

being shown in the coat, giving a pure white cat. W (white coat) is dominant to

w (non-white coat). No matter what other alleles the cat might have for any

coat color, if that cat has a W allele it will have pure white fur, because the W

allele blocks the expression of all other alleles. Draw up the Punnett square for

the mating of two cats, each with the genotype BbWw. What are the expected

ratios of all possible phenotypes in the offspring? (HINT: the answer is NOT

9:3:3:1.)

Each cat can pass on the allele combinations BW, Bw, bW, or bw, so

this seems like a typical dihybrid cross. . .

BW

Bw

bW

bw

BW

BBWW

BBWw

BbWW

BbWw

Bw

BBWw

BBww

BbWw

Bbww

bW

BbWW

BbWw

bbWW

bbWw

bw

BbWw

Bbww

bbWw

bbww

However, if you count, you will note that twelve out of the sixteen

possible genotypes include at least one W allele. W blocks all other

color genes! This means that 3/4 of all cats will be white¡ªregardless

of whether they¡¯re carrying BB, Bb, or bb allele combinations! Only

if a cat has two w alleles will any other color show up. So the answer

is 12:3:1 ¡ª you¡¯d expect twelve white cats (__W_) for every three

black cats (B_ww) for every one brown cat (bbww).

Questions 13-20 have to do with human ABO blood groups. Recall that there are

three alleles of one gene that determine your ABO group: IA, IB, and i. IA and IB

are codominant to each other, and both are dominant to i.

13. In 2003, Mitzi Vanderbeek, who has type O blood, went to a Kid Rock

concert, got invited backstage. . . and nine months later gave birth to a bouncing

baby boy, who has type A blood. What¡¯s the baby¡¯s genotype?

If Mitzi has type O blood, she must have the ii genotype, and she could

only have passed on an i allele to her baby. Her baby has type A

blood, so he must have received the dominant IA allele from his

father, because his mother doesn¡¯t have one. So the baby must have

the genotype IAi.

14. Mitzi wasn¡¯t really sure who the father was, but the men who were, uhh, in

the right place at the right time, if you know what I mean, were Kid Rock

himself (type B blood), guitarist Jason Krause (type A blood), keyboardist

Jimmie Bones (type AB blood) and turntable-ist Uncle Kracker (type O blood).

Which of these four could not be the father of the baby? Why not?

The father, whoever he was, must have been carrying at least one IA

allele. That immediately rules out Kid Rock (type B) and Uncle

Kracker (type O), neither of whom can be carrying the IA allele.

15. Suppose that Jimmie Bones, the keyboardist, turned out to be the father of

Mitzi¡¯s baby. What possible blood types could their children have? Draw up the

Punnett square showing the genotypes and phenotypes of their offspring.

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